LTM4658EVPBF datasheet(10/26 Pages) AD

10 / 26 page

LTM4658

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

the default frequency is required, the frequency can be

programmed by tying a resistor from FREQ pin to AGND

with Equation 4.

RFREQ = 568 • f(SW)(–1.08)

(4)

where RFREQ is in kΩ and fSW is the desired switching

frequency in MHz.

The frequency can be programmed to switch from 1MHz

to 3MHz. Table 2 shows the necessary RFREQ value for a

desired switching frequency.

Table 2. SW Frequency vs RFREQ Value

fSW (MHz)

RFREQ (kΩ)

549

274

2.2

243

178

Synchronizing the Oscillator to an External Clock

The LTM4658 switching frequency can also be adjusted

by synchronizing the internal PLL circuit to an external

clock to the MODE/SYNC pin. The synchronization fre-

quency range is 1MHz to 2.6MHz. The LTM4658 oper-

ates in forced continuous mode when synchronized to

an external clock.

Tie the FREQ pin to VIN configures the MODE/SYNC pin

as a clock input. During synchronization, the top power

switch turn-on is locked to the rising edge of the external

frequencysource.Theslopecompensationisautomatically

adapted to the external clock frequency.

At start-up, before the LTM4658 recognizes the external

clock applied to MODE/SYNC, the LTM4658 will switch at

its default frequency of 2MHz. Once the externally applied

clock is recognized, the switching frequency will gradu-

ally transition from the default frequency to the applied

frequency. If the external clock is removed, the LTM4658

will slowly transition back to the default frequency.

The synchronizing clock amplitude should be greater than

1.2V and less than 0.4V with a pulse width greater than

40ns. An internal 200kΩ resistor on MODE/SYNC pin to

capacitors to minimize the voltage droop and overshoot

during a 2.5A (25%) load-step transient.

Multiphase operation will reduce effective output ripple as

a function of the number of phases. Application Note 77

discusses this noise reduction versus output ripple cur-

rent cancellation, but the output capacitance will be more

a function of stability and transient response. The Analog

DevicesLTpowerCAD®designtoolisavailabletodownload

online for output ripple, stability and transient response

analysis and calculating the output ripple reduction as

the number of phases implemented increases by N times.

Modes of Operation

The MODE/SYNC pin either synchronizes the external

switching frequency, or a clock output, to set the PWM

mode. The PWM modes of operation are either pulse-

skipping or forced continuous. See Table 1.

Table 1. LTM4658 Single Phase Configuration

FREQ PIN

CONNECTION

MODE/SYNC

PIN CONNECTION

MODE OF

OPERATION

SWITCHING

FREQUENCY

VIN

Clock Input

Forced

Continuous

External

Clock

VIN

AGND

Forced

Continuous

2MHz Default

VIN

Pulse-Skip

2MHz Default

Resistor

to AGND

Clock Output

Forced

Continuous

RFREQ

Programmed

The LTM4658 operates in forced continuous mode for

low noise or pulse-skipping mode for high efficiency at

light load. The LTM4658 operates in pulse-skipping mode

when both FREQ and MODE/SYNC pins are connected to

VIN. In pulse-skipping mode, switching cycles are skipped

at light load to regulate the output voltage. The LTM4658

defaults to forced continuous in regulation and during

synchronization. During forced continuous mode, the top

switch turns on every cycle and light load regulation is

achieved by allowing negative inductor current.

Setting the Operating Frequency

The operating frequency defaults to 2MHz when the FREQ

pin is left floating or tie to VIN. If any frequency higher than

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